This invention relates to a system of making an olefin derivative from an olefin feed or its precursor material that has a very low contaminant concentration. More specifically, this invention relates to the making of an olefin derivative by removing a vent stream from an olefin derivative unit, separating olefin from the vent stream, and sending the olefin to an olefin reaction unit.
Olefins such as ethylene, propylene, the butenes, and the pentenes are useful in preparing a wide variety of derivative end products. Examples of such end products include polyethylenes, polypropylenes, polyisobutylene and other polymers, alcohols, vinyl chloride monomer, acrylo-nitrile, methyl tertiary butyl ether and tertiary amyl methyl ether and other petrochemicals, and a variety of rubbers such as butyl rubber.
The olefins used in preparing olefin derivative products are typically made by cracking hydrocarbon feedstocks or catalytically converting oxygenate feedstocks. Cracking of hydrocarbon feedstocks can be accomplished catalytically or non-catalytically. Non-catalytic cracking processes are described, for example, in Hallee et al., U.S. Pat. No. 3,407,789; Woebcke, U.S. Pat. No. 3,820,955, DiNicolantonio, U.S. Pat. No. 4,499,055 and Gartside et al., U.S. Pat. No. 4,814,067. Catalytic cracking processes are described, for example, in Cormier, Jr. et al., U.S. Pat. No. 4,828,679; Rabo et al., U.S. Pat. No. 3,647,682; Rosinski et al., U.S. Pat. No. 3,758,403; Gartside et al., U.S. Pat. No. 4,814,067; Li et al., U.S. Pat. No. 4,980,053; and Yongqing et al., U.S. Pat. No. 5,326,465. Catalytic conversion of oxygenate feedstocks to produce olefins are described, for example in, Kaiser, U.S. Pat. No. 4,499,327, Barger, U.S. Pat. No. 5,095,163, and Hoelderich et al., U.S. Pat. No. 4,433,188.
Olefins which are typically used as feedstock in the preparation of derivative end products are supplied at a relatively high purity to the appropriate reaction unit. For example, when the olefin is ethylene and the derivative product is polyethylene, the ethylene is typically supplied at about 99.9 mol %. As another example, when the olefin is propylene and the derivative product is polypropylene, the propylene is typically supplied at about 99.0 wt. %. With advancements in polymerization catalysis, such as single site metallocene catalysts, feedstock requirements have become even more stringent. See, for example, Chang, U.S. Pat. No. 5,238,892; Burkhardt et al., PCT Application No. WO 9212184; and Schreck et al., U.S. Pat. No. 5,280,074. Accordingly, the conventional wisdom in the industry is for olefin derivative producers, particularly the polyethylene and polypropylene producers, to purchase polymer grade ethylene and propylene, and then remove trace contaminants to obtain the desired purity specifications.
Removal of trace contaminants to provide polymer grade ethylene and propylene can, however, be a difficult and expensive task. For example, separating paraffins from olefins having the same carbon number is quite difficult due to the relatively close boiling ranges of the components. When olefin is produced by cracking, particularly by naphtha cracking, additional contaminants are also a problem. For example, Bodart, U.S. Pat. No. 5,432,243, and Debras et al., U.S. Pat. No. 4,861,939, disclose that arsine and carbonyl sulfide (COS) can be problematic in the olefin derivative process unless the contaminants are removed by additional purification equipment. The purification equipment required is generally quite large in scale and quite expensive to operate.
Purvis et al., U.S. Pat. No. 5,981,818, suggest that, contrary to conventional wisdom, it may be possible to produce polyethylene and polypropylene using ethylene and propylene streams which have a higher than convention content of ethane and propane. The so called dilute propylene and ethylene feeds can be used to prepare polypropylene and polyethylene by sending the by-product stream from the polymerization reaction unit back to a cracking unit or a charge gas compressor downstream of the cracking unit.
Recycling by-product streams from the polymerization unit to a cracking unit or a charge gas compressor can bring about additional problems, however. The by-product can contain significant amounts of ethylene and propylene which will either go through the cracker or compressor unchanged, taking capacity from normal feed conversion, or more likely be modified to a different undesirable by-product. In addition, the recycled by-product is likely to contain sufficient amounts of polymerization catalyst to cause the ethylene and propylene to polymerize in either the cracker or the compressor, causing substantial operating problems due to equipment fouling.
In order to reduce problems associated with contaminant build up in the production of olefin derivative product, this invention provides combining the manufacture of a high quality olefin stream to an olefin reaction unit. Using appropriate recycle of vent gas within the olefin reaction unit allows the high quality olefin stream to be used in the olefin reaction process with reduced risk of equipment fouling and catalyst contamination, while maintaining high product quality.
In one embodiment, the invention provides a method of making an olefin derivative from an olefin stream. The method comprises introducing a dilute olefin stream into an olefin reaction unit; forming an olefin derivative product within the reaction unit; removing the olefin derivative product and a vent stream from the olefin reaction unit; separating olefin from the vent stream; and sending the olefin to the olefin reaction unit. Preferably, the dilute olefin stream comprises at least one C2-C5 olefin stream, more preferably at least 80 wt. % ethylene or 80 wt. % propylene. The vent stream will generally comprise olefin and paraffin.
In another embodiment, the invention is directed to a method of making polyolefin from an oxygenate, which comprises contacting an oxygenate feed with a molecular sieve catalyst to form an olefin-containing product, wherein the olefin-containing product contains less than 10 ppm wt. of a contaminant selected from hydrogen sulfide, carbonyl sulfide, and arsine; separating olefin from the olefin-containing product; contacting the separated olefin with a polyolefin forming catalyst to form a reaction stream comprising polyolefin and a vent gas, wherein the vent gas comprises unreacted olefin; and contacting at least a portion of the unreacted olefin with the polyolefin forming catalyst to form polyolefin. Preferably, the separated olefin is contacted with the polyolefin forming catalyst without having the contaminant previously separated therefrom. In an alternative arrangement, the at least a portion of the unreacted olefin is contacted with the polyolefin forming catalyst without having the contaminant previously separated therefrom.
In another preferred embodiment, the olefin is separated from the olefin-containing product without subsequently separating paraffins from olefins of the same carbon number. It is particularly desirable that ethylene is separated from the olefin-containing product without subsequently separating ethane from the ethylene, and that propylene is separated from the olefin-containing product without subsequently separating propane from the propylene.
In yet another preferred embodiment, the olefin reaction unit includes a polyolefin reactor. The reactor can contain a metallocene catalyst or a Ziegler Natta catalyst.
In another preferred embodiment, the olefin is made from an oxygenate by contacting the oxygenate with a molecular sieve catalyst in which the molecular sieve catalyst is a zeolite or non-zeolite catalyst. Non-zeolite catalysts are preferred. An example of a non-zeolite catalyst includes one which comprises a silicoaluminophosphate molecular sieve. The silicoaluminophosphate molecular sieve is preferably selected from the group consisting of SAPO-5, SAPO-8, SAPO-11, SAPO-16, SAPO-17, SAPO-18, SAPO-20, SAPO-31, SAPO-34, SAPO-35, SAPO-36, SAPO-37, SAPO-40, SAPO-41, SAPO-42, SAPO-44, SAPO-47, SAPO-56, the metal containing forms thereof, and mixtures thereof The oxygenate is preferably selected from the group consisting of methanol; ethanol; n-propanol; isopropanol; C4-C20 alcohols; methyl ethyl ether; dimethyl ether; diethyl ether; di-isopropyl ether; formaldehyde; dimethyl carbonate; dimethyl ketone; acetic acid; and mixtures thereof
In still another embodiment, the invention concerns a process for producing polyethylene. The process comprises contacting an oxygenate containing feed with a molecular sieve catalyst to form an ethylene containing product stream; introducing at least a portion of the ethylene containing product stream into a polyethylene reaction unit to form polyethylene; forming polyethylene within the polyethylene reaction unit; removing the polyethylene product and a vent stream from the polyethylene reaction unit; separating ethylene from the vent stream; and sending the ethylene to the polyethylene reaction unit. Preferably, the ethylene sent to the ethylene reaction unit has a hydrogen sulfide content of not greater than 10 ppm wt., a carbonyl sulfide content of not greater than 10 ppm wt., and an arsine content of not greater than 10 ppm wt. It is also preferred that the ethylene sent to the polyethylene reactor is separated from the ethylene containing product stream without subsequent removal of ethane from ethylene.
In another preferred embodiment, the invention is to a process for producing polypropylene. The process comprises contacting an oxygenate containing feed with a molecular sieve catalyst to form a propylene containing product stream; introducing at least a portion of the propylene containing product stream into a polypropylene reaction unit to form polypropylene; forming polypropylene within the polypropylene reaction unit; removing the polypropylene product and a vent stream from the polypropylene reaction unit; separating propylene from the vent stream; and sending the propylene to the polypropylene reaction unit. Preferably, the propylene sent to the polypropylene reaction unit has a hydrogen sulfide content of not greater than 10 ppm wt., a carbonyl sulfide content of not greater than 10 ppm wt., and an arsine content of not greater than 10 ppm wt. It is also preferred that the propylene sent to the polypropylene reactor is separated from the propylene containing product stream without subsequent removal of propane from propylene.